A torque converter is a device for transmitting torque from an engine to a transmission through an operating fluid contained inside, and mainly includes: a front cover into which the torque from the engine is inputted; an impeller; a turbine; and a stator. The torque inputted into the front cover is outputted to the transmission through the impeller, operating oil and the turbine. The flow of the operating oil returning to the impeller from the turbine is regulated by the stator.
Further, torque converters are often equipped with a lock-up device disposed between the front cover and the turbine. For example, the lock-up device includes: a disc-shaped piston configured to be pressed onto the front cover; and a damper mechanism disposed between the piston and the turbine. The front cover and the turbine are configured to be mechanically coupled by the lock-up device, and thereby, torque is directly transmitted from the front cover to the turbine.
In such lock-up device, the piston has a friction member, which can be frictionally coupled to the front cover, on the outer peripheral part thereof. Further, the piston divides the space produced between the front cover and the turbine into a first hydraulic chamber on the front cover side and a second hydraulic chamber on the turbine side, and is axially movable by differential pressure between the first hydraulic chamber and the second hydraulic chamber (e.g., see Japan Laid-open Patent Application Publication No. JP-A-2005-344846).
In the lock-up device, when the operating oil is drained from the first hydraulic chamber, the hydraulic pressure in the second hydraulic chamber becomes higher than that in the first hydraulic chamber, and the piston is moved towards the front cover. The friction member mounted to the piston is then pressed onto the front cover, and this produces a torque transmitted state that the clutch is engaged (clutch-on). On the other hand, when the operating oil is supplied to the first hydraulic chamber and the hydraulic pressure in the first hydraulic chamber becomes higher than that in the second hydraulic chamber, the friction member is moved towards the turbine and is separated away from the front cover. The clutch is thereby disengaged and this produces a state that transmission of torque is cut (clutch-off).